The invention relates to a low-pressure gas discharge lamp comprising a discharge vessel and at least two capacitive coupling structures which are spatially separated from one another, said discharge vessel having a small diameter of preferably 5 mm or less, while each coupling structure is formed by at least a cylindrical tube of dielectric material whose outer surface is provided with a metallization. Known gas discharge lamps consist of a vessel with a filling gas in which the gas discharge takes place, usually with two metal electrodes fixedly sealed in the discharge vessel. A first electrode supplies the electrons for the discharge, and the electrons are removed to the external current circuit again through the second electrode. The supply of the electrons usually takes place by means of glow emission (hot electrodes), but it may alternatively be generated through emission in a strong electric field or directly through ion bombardment (ion-induced secondary emission) (cold electrodes).
In an inductive mode of operation, the charge carriers are directly generated in the gas volume by an electromagnetic AC field of high frequency (typically higher than 1 MHz in the case of low-pressure gas discharge lamps). The electrons move along closed trajectories within the discharge vessel, and conventional electrodes are absent in this mode of operation. Capacitive coupling structures are used as the electrodes in a capacitive mode of operation. These capacitive electrodes are usually formed from insulators (dielectrics) which are in contact at one side for the gas discharge and which are connected to an external current circuit with electrical conduction (for example by means of a metallic contact) at the other side. An AC electric field is formed when an AC voltage is applied to the capacitive electrodes, and the charge carriers move along the relevant linear electric fields. In the high-frequency range (f greater than 10 MHz), capacitive lamps resemble inductive lamps because the charge carriers are generated throughout the entire gas volume also in the former case. The surface properties of the dielectric electrodes are of minor importance here (so-called xcex1-discharge mode). At lower frequencies, the capacitive lamps change their mode of operation, and the electrons important for the discharge must be emitted originally at the surface of the dielectric electrode and must be multiplied in a so-called cathode drop region so as to keep the discharge going. The emission behavior of the dielectric material, therefore, is a determining factor for the lamp function (the so-called xcex3-discharge mode).
It is advantageous in a number of applications to have available fluorescent lamps of small diameter (preferably 5 mm or less) and as great as possible a quantity of light per unit lamp length (lumens per cm). Furthermore, most areas of application require a high switching stability of the lamp. This is true in particular, for example, in the case of a background illumination for a liquid crystal display. Hot-cathode lamps do not fulfill the above conditions, on the one hand for constructional reasons, and on the other hand because a small diameter of this type of lamps leads to an intensified blackening of the inner surface of the discharge vessel, which in its turn reduces lamp life.
Until recently, fluorescent lamps of small lamp diameter (5 mm or less) were found to be possible only in the form of cold-cathode lamps or in the form of capacitive gas discharge lamps with an operational frequency above 1 MHz. Cold-cathode lamps can be operated at low frequencies (30 to 50 Hz) and accordingly show only a small electromagnetic radiation. The discharge current in this type of lamp, however, is strongly limited (to a maximum of approximately 10 mA). This is caused on the one hand by a strongly intensified sputtering of the electrode material in the case of higher discharge currents. On the other hand, current limitation is necessary for preventing the electrode being heated locally so strongly that thermal emission occurs, which also leads to a strongly intensified cathode sputtering. The electrode material removed through dissolution will deposit itself in the discharge vessel, which leads to a fast blackening of the lamp.
In a capacitive discharge lamp with an operating frequency f greater than 1 MHz, the high operating frequency in combination with a high current density in the lamp (strong current, small lamp diameter) leads to a strong electromagnetic radiation. Large-scale measures have to be taken in order to limit this electromagnetic radiation. Since the power is capacitively coupled, the operating frequency is limited in downward direction (to approximately 1 MHz) by the capacitance of the coupling surface.
EP-A-1 043 757 describes a gas discharge lamp with a capacitive coupling structure. The object here is to supply the gas discharge lamp with the capacitive coupling structure from the public mains for private domestic use without a circuit with starter electronics. This can be achieved, according to this publication, through a suitable choice of dielectric saturation polarization and an effective surface area of the dielectric. This publication does not relate to a gas discharge lamp with a diameter of preferably 5 mm or less and with an accompanying high light output.
Investigations have shown that, as regards the dielectric, a certain ratio between the thickness of the dielectric and the product of the dielectric constant and the frequency can be of importance for obtaining a low-pressure gas discharge lamp with a high light output and with a small diameter, preferably smaller than 5 mm. The gas discharge lamp may suitably be composed of a transparent discharge vessel with a usual filling gas, and may be operated with a frequency f of an AC supply source. The material of the discharge vessel and the filling gas may be chosen so as to correspond to the desired spectrum of the generated radiation. In particular, the discharge vessel may be provided with a fluorescent layer, so that the lamp emits radiation in a certain frequency range (for example in the UV range). At least two mutually separated capacitive coupling structures are present. The dielectric may be composed of one or several layers. The lamp is suitable for operating with a discharge current greater than 10 mA, in which case only a small electromagnetic radiation will occur. The fields of application of such a gas discharge lamp are wide. An important application is, for example, the use as a background illumination of a liquid crystal display.
The invention relates to the latter type of gas discharge lamps. To achieve a practical applicability, however, further electrical, mechanical, and thermal problems are to be solved. The capacitive coupling structure formed by a cylindrical tube of dielectric material in the gas discharge lamp described in EP-A-1 043 757 is provided with a metallization, for example an electrically conducting silver paste. An electrical conductor is soldered to this layer for connection to an external current source. Such an electrical contacting, however, is problematic and not suitable for mass manufacture.
A first object of the invention is to provide a solution to this problem. According to the invention, this is achieved in that the electrical connection to each coupling structure is formed by a resilient clamping element with an internal diameter smaller than the external diameter of the cylindrical tube of dielectric material, which resilient clamping element consists of a temperature-resistant material which can be easily soldered and/or welded and which resilient clamping element surrounds at least a portion of the metallization of the cylindrical tube of dielectric material with clamping force so as to form a large number of contact points, while an end portion of the resilient clamping element facing away from the discharge vessel is fastened to an electrically conducting wire by means of welding or soldering.
Such a contacting is mechanically and electrically reliable and is suitable for mass manufacture. The resilient clamping element surrounding the dielectric coupling structure may be constructed in various ways. A possible construction is, for example, a clamping helical spring which is provided with contact around the metallization of the cylindrical dielectric tube and to which an electrical conductor is soldered.
In a preferred embodiment of the invention, the resilient clamping element is formed by a resilient closing clip surrounding the coupling structure with ribs extending in longitudinal direction of the coupling structure, two ends of the closing clip being interconnected by a clamping piece provided with a slot, while the electrically conducting wire is fastened to said clamping piece.
Not only does this construction lead to a highly reliable electrical contacting and is it highly suitable for mass manufacture, but the further advantage is obtained that the ribs of the closing clip guarantee a favorable heat removal, which benefits a long lamp life.
The resilient closing clip may be made, for example, from copper, brass, or spring steel. The clamping piece provided with a slot may preferably be formed from copper or brass, to which the electrical conductor can be readily fastened.
The low-pressure gas discharge lamp according to the invention is highly suitable for being accommodated in a housing which forms a reflector. The lamp is then highly suitable for use as a background illumination for a liquid crystal display.
Favorably, the reflector may then be formed as an elongate channel of aluminum, the end portions of the lamp comprising the coupling structures and the closing clips being encapsulated in a heat-conducting but electrically insulating synthetic resin inside the end portions of the reflector.
A particularly good heat removal is obtained with a synthetic resin consisting of polyurethane filled with 50% aluminum trihydrate.